Very fine fibres from polymer solutions, often referred to as nanofibres, are useful in a wide variety of applications, including filter media, tissue-engineering scaffold structures and devices, fibre-reinforced composite materials, sensors, electrodes for batteries and fuel cells, catalyst support materials, wiping cloths, absorbent pads, post-operative adhesion preventative agents, smart-textiles as well as in artificial cashmere and artificial leather.
Electrostatic spinning of fibres was first described in U.S. Pat. No. 692,631. In principle, a droplet of polymer solution or melt is placed in a high electric field. The repulsion between the induced like-charges in the droplet compete with the surface tension of the liquid and when sufficiently strong electric field is applied (typically 0.5-4 kV/cm), the electrostatic forces overcome the surface tension of the fluid and a jet of polymer solution or melt is ejected from the droplet. Electrostatic instability leads to rapid, chaotic whipping of the jet, leading, in turn, to fast evaporation of the solvent as well as stretching and thinning of the polymer fibre that is left behind. The formed fibres are then collected on a counter electrode, typically in the form of a nonwoven web. The collected fibres are usually quite uniform and can have fibre diameters of several micrometers, down to as low as 5 nm.
The technical barriers to manufacturing large amounts of nanofibres through electrospinning include low production rates and the fact that most polymers are spun from solution. On average, solution based electrospinning, using needle spinnerets, have solution throughput rates on the order of 1 ml per hour per needle. Fibres with diameters in the range of 50 to 100 nm are typically spun from solutions with relatively low concentrations, 0.5-10 wt % depending on polymer type and molecular weight. This means that, assuming a polymer density of around 1 g/ml, the typical solids throughput rate of a needle-based electrospinning process is 0.005 g to 0.01 g of fibre per hour per needle. If one extends this calculation, producing a nanofibre web with a planar density of 80 g/m2 at a rate of 5 m2/s will require a minimum of 40 000 needles. In addition to the requirement for such large numbers of needles, electrical field interference between the different needles also limits the minimum separation between them and furthermore, continuous operation of needle-based spinnerets requires frequent cleaning of the needles as polymer deposits block the spinnerets.
Although the electrospinning process is relatively cost effective on a laboratory scale, the low rates of fibre throughput on single-needle setups make production at industrial volumes prohibitively expensive for most commodity applications like filtration and absorbent textiles. By increasing production rates, the cost can be dramatically lowered, broadening the scope of application for electrospun nanofibres and opening the door to the development of new technologies.
Formhals already tried to increase electrospinning production rates in 1934 (U.S. Pat. No. 1,975,504) by using multiple cogwheel sources. In later designs, he used multiple needle setups (U.S. Pat. No. 2,109,333) which have since become the first obvious approach to increase electrospinning production rates in the laboratory. The multiple needle approach might appear straightforward, but it is often inconvenient due to system complexity and the high probability of needle clogging.
In more recent times, different approaches have been proposed. Reneker et al. (PCT WO 00/22207) describe a process in which nanofibres were produced by feeding fibre-forming solution into an annular column, forcing a gas through the column in order to form an annular film which was then broken up into numerous strands of fibre-forming material.
Kim (PCT WO 2003/03004735) designed a complex multiple nozzle block system in which the spinning solution is controlled through gas flow.
Upward needleless electrospinning of multiple nanofibres proposed by A. L. Yarin, E. Zussman, Polymer 45 (2004) 2977-2980 uses a two-layer system, with the lower layer being a ferromagnetic suspension and the upper layer a polymer solution. When a permanent magnetic field was applied to the system, steady vertical spikes of the magnetic fluid pushed up through the interlayer interface and the free layer of the polymer solution. When a strong electric field was applied across the system in this state, multiple electrospinning jets initiated from the spike tips, leading to high rates of fibre production. When the jet packing density was compared to a multiple needle setup, a twelve fold enhancement in production was calculated. The needle-less process also avoids potential problems with clogging of needles. Potential drawbacks of the system include compatibility issues between the magnetic suspension and the polymer solution and the risk of contamination of the fibres from the fluid.
A special design for a melt-electrospinning multiple-needle nozzle pack was proposed by Chun and Park (PCT WO 2004/016839). However, except for the additional polymer melting components, the design did not differ significantly from previous multiple needle designs.
Karles et al. (PCT WO 2004/080681) describe various designs for higher throughput spinning and special counter-electrodes for the formed fibres but none of the multiple needle and spiked hairbrush-type spinning sources vary significantly from the needle and cogwheel sources already described by Formhals in the 1930s.
Improving on his 2003 design, Kim, together with Park, designed an upward spinning nozzle block with overflow-removing nozzle blocks and additional air-flow nozzles (PCT WO 2005/090653). In this design, the spinning nozzles consist of three concentric tubes. The inner tube supplies the spinning solution, the intermediate tube serves to remove excess non-spun solution when it overflows, and the outer tube creates a gas pocket around the spinning jet, reducing the effect of electrostatic repulsion that jets have on neighbouring jets. This design was incorporated into a subsequent patent describing the formation of continuous yarns from electrospun nanofibre webs (PCT WO 2005/073442).
Andrady et al. designed a system (PCT WO 2005/100654) consisting of a rotating tube through which the spinning solution is pumped to several jet outlets on the surface of the tube. The electrospun fibres are then collected on another rotating tube which is placed around the outside of the inner spinning tube. Despite this and additional complexity related to gas flows through the system, the spinning solution was pumped at a rate of approximately 1.5 ml/h, which is not much higher than the typical 1.0 ml/h flow rate used with a single-jet setup. Although the system is claimed to be for high throughput electrospinning, it rather embodies a special case of the laboratory-scale rotating drum method of fibre collection.
Andrady and Ensor subsequently designed another process, in which the polymer solution is pumped into a single, box-like container with 2 to 100 needle-like exits on the one side (PCT WO 2006/043968). This design is very similar to that used by NanoStatics (www.nanostatics.com). In both cases, high throughput of fibres is achieved, but the large dead volume of fluid behind the needles results in poor control of the flow rates at each needle. This in turn can lead to droplets and sputtered polymer fragments in the final fibre web.
A recent design by Beetz et al. (PCT WO 2006/047453) consists of a combination of high-pressure atomization and simultaneous electrospraying or electrospinning of a fluid. In essence, the spinning fluid is forced, under high pressure, through a small-diameter (<1 mm) tube, whilst applying a high voltage to the fluid.
Multiple jets on a porous tubular surface by Dosunmu et al., Nanotechnology 17 (2006) 1123-1127 describes the use of a polymer solution which was electrified and pushed by air pressure through the walls of a porous polyethylene tube. Multiple jets formed on the porous surface and electrospun into nanofibres. The production rate from the tube was approximately 250 times faster than a typical single jet. Further work still needs to be performed, but initial calculations indicate potential production rates the order of 4.2 g/min per meter length of porous tube. Although this method shows a lot of promise, some restrictions are placed on the spinnability of certain polymers by solution parameters like viscosity and conductivity.
The most significant high throughput electrospinning system at present is known as NanoSpider (http://www.nanospider.cz/). In this process, the fibre forming polymer solution is placed in a dish and a conductive cylinder is slowly rotated through the spinning solution, forming a thin layer of solution on the surface of the cylinder. When a sufficiently high voltage is applied between the spin-cylinder and the counter-electrode placed 10-20 cm above the cylinder, hundreds of jets initiate off the surface of the cylinder and electrospin onto the target. The laboratory-scale configuration of NanoSpider, depending on the polymer, has a productivity of about 1 g/min.
Japanese patent JP3918179 describes a process in which bubbles are continuously generated on the surface of a polymer solution by blowing compressed air into the solution through a porous membrane, or through a thin tube. High voltage is applied between the polymer solution and a counter-electrode plate. When the voltage is high enough, electrospinning jets are formed on the bubbles in the polymer solution and the fibres that form are collected on the counter-electrode. This disclosed process requires that the bubbles in the polymer solution be formed in high volumes and that they subsequently burst very rapidly. Although it is stated that any dissolvable polymer can be used, and any suitable solvent including various organic solvents, it is well known that most organic solvents do not readily form foams. The bubbles formed in such organic solutions will therefore be very short-lived. Additionally, although the patent claims general applicability to organic solutions, the given examples demonstrate spinning only with polymer solutions in water, 2-propanol and acetone. The patent also describes the requirement that the counter-electrode be placed at a suitable distance from the foam since droplets of spin solution that are created by the constantly bursting bubbles can spatter onto and destroy the already formed fibres on the counter-electrode.